If the stove or fireplace is continuously smoking then it is very unlikely to be downdraught as chimney downdraught usually occurs in puffs rather than a continuous stream of smoke. There are a number of possible causes for a smoking fireplace or stove. These are:
Unswept / blocked chimney.
Competition with another chimney or extractor.
Excessive fireplace opening size in relation to the flue size.
Incorrect size of chimney pot.
Unlined / cold (uninsulated) chimney.
Pressure difference around building.
Insufficient chimney height and therefore inadequate draw.
Any one of these or a combination can contribute to a smoking fireplace or stove.
Lets try to eliminate each cause:
Unswept or blocked chimney
Make sure that your chimney has been swept recently and have a look up it (and down if necessary) with a torch. If there is a lots of soot/tar deposits or a birds nest or fallen bricks, etc then this may be part of the problem.
Sweep and clean the flue pipe from the stove and the internal parts of the stove like the baffle plate. Make sure that there are no leaks or blockages in the flue pipe.
Competition with another chimney or extractor
If there is another chimney in the house then this may have a stronger draw than the smoking chimney. It will be pulling air from any source and one of these sources may be the smoking chimney. Block off any unused chimneys (you should block these off anyway - please read about capping unused chimneys) so that they cannot draw air from the house.
If you have a stove on another chimney then close the air vents and door.
If you have a fireplace on another chimney then close the back baffle if it has one and block it temporarily if not.
If the smoking problem improves then the other chimneys are partly to blame as they are pulling air down the smoking chimney. Cap unused chimneys.
If you have any extractors (for example a cooker hood fan) then make sure that these are turned off. An extractor sucks air out of your house which means that it requires an air supply from outside the house. If you ensure that there is an air supply (eg ventilation) near the extractor then it will draw most or all the air it needs from there and thus there will be fewer draughts and it will not compete with the chimneys in your house.
If the smoking improves when the extractor is off then that is at least party to blame for your smoking fireplace or chimney. Consider installing adequate ventilation close to the extractor.
Open a window in the room with the smoking fireplace or chimney. If that room does not have a window then open a window or external door in the room closest to the room with the smoking fireplace or stove and make sure that the doors between these two rooms are open.
Using something non-combustible block off the top third of the fireplace opening.
If the smoking improves then your fireplace opening may well be too large. This is a very common cause of a smoking fireplace. Consider adding a recessed metal plate at the top of the fireplace opening to make it smaller or install a stove. You can read more and calculate the correct fireplace opening size on our fireplace opening size page.
Generally the higher a chimney is the better the draw. You can read more on our chimney height page ***. You could consider raising the height of your chimney by adding another few courses of bricks or by adding a taller chimney pot.
Restrictive chimney pot or insert
However large your chimney the chimney pot may well be too small. If the pot you have is narrow or your fireplace opening large then consider fitting a wider chimney pot. Sometimes it is not the chimney pot that is restrictive but the cowl or chiney pot insert. Many people fit a Hood Top Insert. These are not suitable for use with a fireplace or stove - one is pictured below.
Pressure difference outside your house
If you have a strong prevailing wind then outside on one side of your house there may be a higher pressure than on the other. This pressure difference can result in air being sucked down the chimney and out to the lower pressure area. There is not much you can do about this if this is the case although pressure difference is relatively rare. Installing an Exhausto Chimney Fan may be the only solution.